**4. Electrical characteristics of Schottky diodes based on semiinsulating CdTe single crystals**

the spectral distribution of the charge collection efficiency (the value determining the energy resolution of the detector) is obtained by dividing the detection efficiency *η*(*hν*) by the absorption capacity of the crystal *A*(*hν*) *=* 1 - exp(−*α*γ*d*). **Figure 4b** shows the collection efficiency

rent is the same as at voltage of 60 V for crystal thickness of 1 mm (3·10−8 A). As seen, when

tal thins, the charge collection efficiency is significantly improved reaching a level of 97–98%. With the thickness of 0.25 mm the charge collection efficiency is above 97% throughout the

ning of the crystal is achieved with a significant decrease in the efficiency of detection (registration) in the range of high-energy photons. A significant increase in the energy resolution can be achieved by improving the quality of CdTe crystals and, thus, increasing the lifetime of charge carriers. Our calculations show that with increasing the electron lifetime by the order of magnitude from 3 × 10−6 s to 3 × 10−5 s, for the crystal thickness of 0.25 mm at voltage that corresponds to the current 3 × 10−8 A (10.3 V) the energy resolution in the spectra of all isotopes

CdZnTe and CdMnTe-based X/γ-rays Ohmic detectors can have electrical characteristics both similar to presented above and different. This is largely due to the choice of contacts material, the treatment of the crystal surface before contacts fabrication, conditions of

characteristics are linear, but at higher bias a superlinear increase in current is observed approximately the same extent at different temperatures. The fact that the voltage dependence of difference between the measured current and a linearly extrapolated current is quadratic, which indicates that the observed supernular growth of current is due to by space charge limited current (SCLC) according to the Mott-Gurney law [17]. The activation energy of the conductivity caused by the equilibrium holes (at *V* = 10 V) equals to 0.39 еV. Attention is drawn to the fact that the energy of acceptor trap at the formation of the SCLC in the Ni/CdMnTe contact (at *V* > 200 V), equal also 0.39 еV, that is, impurity (or defect), responsible for electrical conductivity of material and trap of injected charge carriers clearly have the same nature. Therefore, the same activation energy for the current of equilibrium holes and the current surplus of equilibrium current confirms the fact that SCLC in the Ni/CdMnTe/Ni detector is formed by the injection of majority carriers (holes) from the metal, not by the tunnel injection of minority carriers (electrons) as in the case of Pt/CdTe/Pt detectors discussed above.

Te (*x* = 0.1) n-type crystals with gold Ohmic contacts show other features of superlin-

ear current growth at high voltages. At voltages, lower ~ 10 V *I-V* characteristic are linear, but at higher bias, the superlinear increase is observed. However, the voltage of deviation from the linear law is 10–20 V regardless of temperature. It turns out that the current of equilibrium electrons and the excess current in the Au/CdZnTe/Au detector are growing approximately equally with the temperature. This is confirmed by the fact that the thermal activation energy of the crystal Cd0,9Zn0,1Te is 0.74 eV, and the thermal activation energy of the excess current at

the crystal thickness is 4 mm, the charge collection efficiency *η*<sup>o</sup>

whole spectral range. However, increasing the charge collection efficiency *η*<sup>o</sup>

post-deposition treatment. At low voltage applied to the p-Cd1-xMn<sup>x</sup>

*hν* < 100 keV is 90%, while for *hν* ≈ 1 MeV *η*<sup>o</sup>

is higher than 99% [20] (**Figure 4c**).

(*hν*) calculated for the different thicknesses of CdTe and voltages at which the cur-

(*hν*) in the photon energy

Te (*x* = 0.3) crystal *I-V*

(*hν*) with thin-

(*hν*) is reduced to 77% (**Figure 4b**). When the crys-

curves *η*<sup>o</sup>

36 New Trends in Nuclear Science

Cd1-xZn<sup>x</sup>

The section deals with electrical characteristics of Ni/CdTe/Ni X/γ-rays detectors with Schottky diodes based on high-resistivity CdTe single crystals (*ρ* ~109 Ω·сm (300 К)).

The theoretical analysis of experimental results allows identifying and explaining the essential features of the charge transport mechanisms depending on the resistivity of the material and the parameters of the diode structure, in particular the concentration of uncompensated impurities (defects) and the height of the potential barrier on Schottky contact [21]. According to the Sah-Noyce-Shockley theory, the current through the diode is determined by the integration of the generation-recombination rate over the whole space charge region (SCR) width [22].

of the \"generation-recombination rate over the whole space charge region (\"SCR\") width [22].

$$I\_{\mathbf{x}^{\star}} = Aq \int\_{0}^{\mathbf{\tilde{\nu}}} \frac{n(\mathbf{x}, \mathbf{\nu})p(\mathbf{x}, \mathbf{\tilde{\nu}}) - n\_{i}^{2}}{\tau\_{p\mathbf{\tilde{\nu}}}[n(\mathbf{x}, \mathbf{\tilde{\nu}}) + n\_{i}] + \tau\_{m}[p(\mathbf{x}, \mathbf{\tilde{\nu}}) + p\_{i}]} \, d\mathbf{x},\tag{6}$$

where *А* is the diode area, *q* electron charge, *W* is the width of the SCR, *n*(*x,V*) and *p*(*x,V*) - are the concentrations of charge carriers in the conduction and valence bands, respectively, *τ*no and *τ*po are the effective lifetimes of electrons and holes in the SCR, and the quantities *n*<sup>1</sup> = *N*<sup>c</sup> exp(−*E*<sup>t</sup> /*kT*) and *p*<sup>1</sup> = *N*<sup>v</sup> exp[−(*E*<sup>g</sup> - *E*<sup>t</sup> )/*kT*] are determined by the depth of the generation-recombination level *E*t . The results of calculations of the *I*–*V* characteristic, by using formula (6) show that the model of generation-recombination processes in the SCR adequately describes not only the current dependence on the voltage, but also the temperature induced variations in the Ni/p-CdTe Schottky diode *I*–*V* characteristic: (1) The reverse current, which has a generation origin, cannot vary in a wide range of the material resistivity *ρ* since this current is governed by the carrier lifetime and by the thickness of the SCR, which have no direct relation with a value of *ρ*. (2) In the region of low forward biases, where the dependence *I* ∝ exp(*qV*/2*kT*) – 1 holds, the current is governed by the same parameters and, therefore, is also only slightly *ρ*-dependent. (3) As *ρ* increases, the Fermi level recedes from the valence band; that is, *∆μ* increases at the same time as *φ*<sup>0</sup> decreases. In this case, the part of the forward branch, where the forward current is proportional to exp(*qV*/2*kT*), is increasingly restricted from above, as is observed in the experimental curves.

The Ni/CdTe/Ni diode structure with Schottky and near-Ohmic contacts at the CdTe(111)A and CdTe(111)B surfaces of semi-insulating CdTe single crystals (*ρ =* (2–4)·109 Ω·сm) demonstrates absence of rectification properties at bias voltages lower than 6–7 V, which can be attributed to a very high resistance of the CdTe substrate, that is, the voltage drop across the bulk part of the crystal should be taken into account. It should be noted that consideration of the voltage drop has strongly modified the shape of the forward *I-V* characteristic of the studied diode structure (**Figure 5b**) [21, 23]. A sharp increase in the current at higher forward bias voltages is attributed to the injection of minority carriers from the Schottky contact into the neutral part of the crystal and the modulation of its electrical conductivity, which is confirmed by the results of calculations. It should be emphasized that the Ni/CdTe/Ni detectors with Schottky and near-Ohmic contacts demonstrates low reverse current (~10−9 A/cm2 at 300 К) at high reverse bias due to significant bending on the Ni/CdTe Schottky contact and low enough level of minority carrier injection from the near-Ohmic CdTe/Ni contact into the neutral part of the diode structure. It should be noted, the generation-recombination Sah-Noyce-Shockley theory analytically describes the *J-V* characteristic of the diode structure at different temperatures (**Figure 5b**) [24, 27]. Analysis of the voltage dependence of the differential resistance *R*diff shows, at forward connection decreases with increasing in the low-bias region (**Figure 5c**). In the voltage range *V* = 1–3 V, the *R*diff saturates, which means that the energy barrier is practically compensated by applied voltage and further voltage drop takes place across the bulk part of the diode structure. With further increasing the forward bias voltage, a sharp decrease in *R*diff is observed. The value of *R*diff becomes 2–3 orders of magnitude less than resistance of the bulk part of the diode *R*<sup>s</sup> . Such lowering of *R*diff is explained by injection of electrons (minority carriers) from the forward-biased Schottky contact into the bulk part of the crystal and modulation of its resistance (**Figure 5a** and **c**). Indeed, at higher forward voltage the barrier *φ*<sup>o</sup> lowers and electron injection in the bulk part of the crystal is increasingly enhanced.

**5. Detection efficiency of CdTe based X/γ-ray detector**

biased. The dashed straight line shows the resistance of the bulk part of the diode structure *R*<sup>s</sup>

bottom, respectively. The recombination (*J*rec), generation (*J*

and (3–5)·1010 Ω·сm, respectively. The band gap of the crystals *E*<sup>g</sup>

The parameters of crystal and diode structure significantly affect the quantum detection efficiency and energy resolution of detectors based on semi-insulating CdTe and Cd0,9Zn0,1Te crystals with Schottky diode. In such crystals with deep levels of impurities (defects) in the band gap, the density of the space charge and the intensity of the electric field grow rapidly near the crystal surface, enhanced with the increase in the degree of compensation of the semiconductor (in contrast to the Schottky diodes on the semiconductor with shallow impurities levels). Minority charge carriers play an insignificant role in the formation of space charge, despite the presence of an inverse layer near the surface of the semiconductor. In spite of the features of the formation of SCR in the Schottky diodes based on self-compensating semiconductors (which are CdTe and Cd0,9Zn0,1Te crystals, doped with Cl or In), the difference between the value of the SCR width, as determined by the solution of the Poisson equation, and by means of the known the formula for the Schottky diode does not exceed 15–16% even with a high compensation degree, that is, the width of the SCR is quite accurately determined by the concentration of uncompensated impurities. The charge collection efficiency in X/γ-ray detectors with a Schottky diode essentially depends on the carrier lifetime *τ*. It is important for practice that the charge collection efficiency is noticeably lower than 1 when the lifetime is less than 10−8 s, whereas to provide practically the total charge collection (99%) in the Ohmic detector the carrier lifetime should be equal to or exceed ∼10−6 s. The resistivity of CdTe and CdZnTe crystals under study at room temperature are (2–3)·109

**Figure 5.** (a) The energy diagram of the forward and reverse biased Ni/CdTe/Ni diode structure shown at the top and

characteristics of the Ni/CdTe/Ni structure at different temperatures. The circles show the measurement results; the lines are the results of calculations by Eq. (6). (c) Differential resistance of the detector in a wide range of forward and reverse

gen) an injection (*J*

Mechanisms of Charge Transport and Photoelectric Conversion in CdTe-Based X- and Gamma-Ray Detectors

1.48 eV, for Cd0,9Zn0,1Te *E*<sup>g</sup> = 1.53 eV at room temperatures. Our studies of the relaxation curves of the rise and decay of the photocurrent excited by rectangular pulses of semiconductor laser (*λ* = 782 nm) showed that the lifetimes of the charge carriers in the SCR and in the neutral part of the CdTe crystal differ significantly. In the case of CdTe crystal with two Ohmic contacts, the lifetimes of electrons amount to a few microseconds, which is consistent with the data presented on the site of Acrorad Co. Ltd. [14]. If the crystal is irradiated through a

for CdTe equals to 1.47–

inj) currents are shown by arrows. (b) *J-V*

http://dx.doi.org/10.5772/intechopen.78504

39

.

Analysis of the reverse *J-V* characteristic at high-bias voltages that is most important and interesting in the application of CdTe diodes as X/γ-ray detectors (*V* < 600–700 V) shows that the reverse current through the diode structure is controlled by the reverse-biased Schottky contact. A sublinear rise in the current (it is typical for the generation charge transport mechanism) corresponds to a gradual increase in the differential resistance (**Figure 5c**). However, on exceeding 600–700 V, the differential resistance decreases increasingly and then steeply decays at similarly to that at forward connection of the diode at voltages higher than a few volts. It can be explained by injection of electrons from the near-Ohmic contact into the bulk of the crystal [21, 23, 24] (**Figure 5a**). With an increase in the current, a fraction of the applied voltage, much like for a forward connection of the device, drops across the neutral part of the crystal and only a small its fraction drops across the near-Ohmic contact on the opposite side of the crystal. Thus, we have come to not at all trivial conclusion that at relatively high reverse bias, the processes in the "Ohmic" contact affect the reverse-biased Schottky contact on the opposite side of the crystal [15, 21, 23, 24]. A decrease in injection of carriers from the near-Ohmic contact in a Schottky diode with Ni/CdTe/Ni electrode configuration is an important way to reduce the leakage current and improve the performance of CdTe based X/γ-ray detectors. On increasing the operating voltage at low-leakage current allows to enhance the detection efficiency of the device especially in the region of high energy of photons.

Mechanisms of Charge Transport and Photoelectric Conversion in CdTe-Based X- and Gamma-Ray Detectors http://dx.doi.org/10.5772/intechopen.78504 39

**Figure 5.** (a) The energy diagram of the forward and reverse biased Ni/CdTe/Ni diode structure shown at the top and bottom, respectively. The recombination (*J*rec), generation (*J* gen) an injection (*J* inj) currents are shown by arrows. (b) *J-V* characteristics of the Ni/CdTe/Ni structure at different temperatures. The circles show the measurement results; the lines are the results of calculations by Eq. (6). (c) Differential resistance of the detector in a wide range of forward and reverse biased. The dashed straight line shows the resistance of the bulk part of the diode structure *R*<sup>s</sup> .

#### **5. Detection efficiency of CdTe based X/γ-ray detector**

The Ni/CdTe/Ni diode structure with Schottky and near-Ohmic contacts at the CdTe(111)A and CdTe(111)B surfaces of semi-insulating CdTe single crystals (*ρ =* (2–4)·109 Ω·сm) demonstrates absence of rectification properties at bias voltages lower than 6–7 V, which can be attributed to a very high resistance of the CdTe substrate, that is, the voltage drop across the bulk part of the crystal should be taken into account. It should be noted that consideration of the voltage drop has strongly modified the shape of the forward *I-V* characteristic of the studied diode structure (**Figure 5b**) [21, 23]. A sharp increase in the current at higher forward bias voltages is attributed to the injection of minority carriers from the Schottky contact into the neutral part of the crystal and the modulation of its electrical conductivity, which is confirmed by the results of calculations. It should be emphasized that the Ni/CdTe/Ni detectors with Schottky and near-Ohmic contacts demonstrates low reverse current (~10−9 A/cm2 at 300 К) at high reverse bias due to significant bending on the Ni/CdTe Schottky contact and low enough level of minority carrier injection from the near-Ohmic CdTe/Ni contact into the neutral part of the diode structure. It should be noted, the generation-recombination Sah-Noyce-Shockley theory analytically describes the *J-V* characteristic of the diode structure at different temperatures (**Figure 5b**) [24, 27]. Analysis of the voltage dependence of the differential resistance *R*diff shows, at forward connection decreases with increasing in the low-bias region (**Figure 5c**). In the voltage range *V* = 1–3 V, the *R*diff saturates, which means that the energy barrier is practically compensated by applied voltage and further voltage drop takes place across the bulk part of the diode structure. With further increasing the forward bias voltage, a sharp decrease in *R*diff is observed. The value of *R*diff becomes 2–3 orders of magni-

by injection of electrons (minority carriers) from the forward-biased Schottky contact into the bulk part of the crystal and modulation of its resistance (**Figure 5a** and **c**). Indeed, at higher

Analysis of the reverse *J-V* characteristic at high-bias voltages that is most important and interesting in the application of CdTe diodes as X/γ-ray detectors (*V* < 600–700 V) shows that the reverse current through the diode structure is controlled by the reverse-biased Schottky contact. A sublinear rise in the current (it is typical for the generation charge transport mechanism) corresponds to a gradual increase in the differential resistance (**Figure 5c**). However, on exceeding 600–700 V, the differential resistance decreases increasingly and then steeply decays at similarly to that at forward connection of the diode at voltages higher than a few volts. It can be explained by injection of electrons from the near-Ohmic contact into the bulk of the crystal [21, 23, 24] (**Figure 5a**). With an increase in the current, a fraction of the applied voltage, much like for a forward connection of the device, drops across the neutral part of the crystal and only a small its fraction drops across the near-Ohmic contact on the opposite side of the crystal. Thus, we have come to not at all trivial conclusion that at relatively high reverse bias, the processes in the "Ohmic" contact affect the reverse-biased Schottky contact on the opposite side of the crystal [15, 21, 23, 24]. A decrease in injection of carriers from the near-Ohmic contact in a Schottky diode with Ni/CdTe/Ni electrode configuration is an important way to reduce the leakage current and improve the performance of CdTe based X/γ-ray detectors. On increasing the operating voltage at low-leakage current allows to enhance the detection efficiency of the device especially in the region of high

. Such lowering of *R*diff is explained

lowers and electron injection in the bulk part of the crystal is

tude less than resistance of the bulk part of the diode *R*<sup>s</sup>

forward voltage the barrier *φ*<sup>o</sup>

increasingly enhanced.

38 New Trends in Nuclear Science

energy of photons.

The parameters of crystal and diode structure significantly affect the quantum detection efficiency and energy resolution of detectors based on semi-insulating CdTe and Cd0,9Zn0,1Te crystals with Schottky diode. In such crystals with deep levels of impurities (defects) in the band gap, the density of the space charge and the intensity of the electric field grow rapidly near the crystal surface, enhanced with the increase in the degree of compensation of the semiconductor (in contrast to the Schottky diodes on the semiconductor with shallow impurities levels). Minority charge carriers play an insignificant role in the formation of space charge, despite the presence of an inverse layer near the surface of the semiconductor. In spite of the features of the formation of SCR in the Schottky diodes based on self-compensating semiconductors (which are CdTe and Cd0,9Zn0,1Te crystals, doped with Cl or In), the difference between the value of the SCR width, as determined by the solution of the Poisson equation, and by means of the known the formula for the Schottky diode does not exceed 15–16% even with a high compensation degree, that is, the width of the SCR is quite accurately determined by the concentration of uncompensated impurities. The charge collection efficiency in X/γ-ray detectors with a Schottky diode essentially depends on the carrier lifetime *τ*. It is important for practice that the charge collection efficiency is noticeably lower than 1 when the lifetime is less than 10−8 s, whereas to provide practically the total charge collection (99%) in the Ohmic detector the carrier lifetime should be equal to or exceed ∼10−6 s.

The resistivity of CdTe and CdZnTe crystals under study at room temperature are (2–3)·109 and (3–5)·1010 Ω·сm, respectively. The band gap of the crystals *E*<sup>g</sup> for CdTe equals to 1.47– 1.48 eV, for Cd0,9Zn0,1Te *E*<sup>g</sup> = 1.53 eV at room temperatures. Our studies of the relaxation curves of the rise and decay of the photocurrent excited by rectangular pulses of semiconductor laser (*λ* = 782 nm) showed that the lifetimes of the charge carriers in the SCR and in the neutral part of the CdTe crystal differ significantly. In the case of CdTe crystal with two Ohmic contacts, the lifetimes of electrons amount to a few microseconds, which is consistent with the data presented on the site of Acrorad Co. Ltd. [14]. If the crystal is irradiated through a semitransparent Schottky contact, the laser radiation is absorbed in a thin near-surface layer of the SCR and lifetimes of carriers are relatively short (10–20 ns).

To determine the concentration of uncompensated impurities in crystals of CdTe and Cd0,9Zn0,1Te, we compared the detection efficiency with irradiation of the crystal by the Оhmic contact side and the Schottky contact side. In the high-energy region of the spectrum, in which the absorption coefficient for X/γ-rays (*α*γ) is small, excitation occurs virtually uniformly over the entire crystal volume and the detection efficiency for a crystal with the Schottky contact is independent of which side of the detector is irradiated, the side of the Schottky contact or the side of the Ohmic contact. This is confirmed experimentally. If a CdTe detector with a Schottky contact is subjected to the radiation of the 137Cs isotope, the peak height at the photon energy 662 keV (*α*<sup>γ</sup> ≈ 0.1 cm−1) is practically the same when different sides of the sample are irradiated, the side of the Schottky contact or the side of the Ohmic contact. If the isotope 55Fe is used (*α*<sup>γ</sup> ≈ 4000 cm−1), the peak height in the case of irradiation of the Schottky contact side is by two orders of magnitude, than that in the case of irradiation of the Ohmic contact side, as is shown in **Figure 6a** (inset) [11]. This is accounted for by the fact that, at *α*<sup>γ</sup> ≈ 4000 cm−1, the effective depth of radiation penetration into the material is smaller than 1 μm, therefore, in the case of irradiation of the Ohmic contact side, a significant portion of electrons, which appeared as a result of γ-photon absorption, do not reach the SCR by diffusion [17]. Evidently, in this case, the peak height greatly depends on the SCR width and, consequently, on the concentration of uncompensated impurities in the semiconductor,

Mechanisms of Charge Transport and Photoelectric Conversion in CdTe-Based X- and Gamma-Ray Detectors

between the concentrations of uncompensated impurities in CdTe and Cd0,9Zn0,1Te crystals, which are used in the fabrication of X/γ-rays detectors. The concentration of uncompensated impurities is ~(1–3) × 1012 cm−3 for CdTe crystals and (1–5) × 10<sup>8</sup> cm−3 (i.e., four orders of magnitude lower) for Cd0,9Zn0,1Te crystals [11]. The low concentration of uncompensated

tors regardless of a fully acceptable resistivity of crystals (> 109 Ω·сm) and the lifetime of

**Figure 6.** (a) Spectra of 133Ba isotope taken with Schottky diode detectors based on CdTe and at *V* = 500 V. The inset shows the emission spectra of the 55Fe isotope measured by a CdTe detector with a Schottky contact under irradiation of different sides of the sample. (b) Detection efficiency spectra of CdTe-based detector with Schottky diode calculated for

. The inset shows the typical 137Cs radioisotope energy spectrum detected by an Ni/CdTe/Ni diode

сm−3) is the reason for the unsatisfactory detectivity of Cd0,9Zn0,1Te detec-

. Thus, there is a significant difference

http://dx.doi.org/10.5772/intechopen.78504

41

. (c) Normalized detection efficiency of different isotopes as

which can be used to determine the value of *N*<sup>d</sup> *– N*<sup>a</sup>

impurities (10<sup>8</sup>

a function of *N*d – *N*<sup>a</sup>

detector.

–109

different concentrations of uncompensated donors *N*d – *N*<sup>a</sup>

charge carriers (> 10−6 s) [10].

Although all crystals had high resistivity and minority carrier lifetime, the diodes showed significant differences in the registration of spectra from 137Cs (662 кеВ), 133Ba (356 кеВ), 57Co (122 кеВ), 241Am (59 кеВ), 55Fe (5,9 кеВ) isotopes. The CdTe detector was a high resolution detector, however the CdZnTe registered the spectra but with lower resolution (**Figure 6a**). At first glance it seems unclear as CdTe inferior in characteristics Cd0,9Zn0,1Te. Obviously, the detecting properties of the diode structure are influenced by other characteristics of the material. There is an assumption that such a parameter is the SCR width of a Schottky diode, which in a compensated semiconductor can be significant. Indeed, in the detector with Schottky diode, the SCR itself is an active area of the detector, and its width, of course, is one of the most important parameters. Therefore, in compensated semiconductor in addition to the high resistivity and long carrier lifetimes that is necessary for high detection efficiency, another mandatory requirement to the concentration of uncompensated impurities in the material (which determines the SCR width of a Schottky diode) is substantiated.

The detection efficiency spectra of CdTe-based crystals with Schottky diode taking into account the drift and diffusion components can be expressed as [25].

$$\eta = \frac{\lambda\_{\text{s}}}{W} \Big[ 1 - \exp\left(-\frac{W}{\lambda\_{\text{s}}}\right) \Big] \Big( \int\_{0}^{\eta} a\_{\text{c}\text{cn}} \exp(-a\_{\text{c}\text{cn}} \cdot x) d\mathbf{x} + \frac{a L\_{\text{s}}}{1 + a L\_{\text{s}}} \exp(-a\mathcal{W}) \Big). \tag{7}$$

**Figure 6b** shows the detection efficiency spectra of CdTe (Cd0,9Zn0,1Te) crystals with Schottky diode at the voltage *V* = 400 V applied to the detector and different concentrations of uncompensated donors *N*<sup>d</sup> *– N*<sup>a</sup> in the material. The results clearly illustrate the fact that the spectra of *η*(*hν*) can significantly be modified when *N*<sup>d</sup> *– N*<sup>a</sup> is changed. If *N*<sup>d</sup> *– N*<sup>a</sup> decreases from 1014 to 1010 cm−3, the detection efficiency of 55Fe (*hν* = 5.9 kеV) and 241Am (*hν* = 59.5 kеV) isotopes vary almost by 3 and 2 orders of magnitude, respectively. At the same decreasing *N*<sup>d</sup> *- N*<sup>a</sup> , the detection efficiency of 57Co (*hν* = 122 kеV) isotope varies within one order of magnitude and the detection efficiency of 133Ba (356 kеV) and 137Cs (662 kеV) isotopes vary relatively weak. An important feature of the results is that the dependences *η*(*N*d–*N*<sup>a</sup> ) for all the isotopes are described by a curve with maximum (**Figure 6c**) [6, 10, 11, 25]. As seen, in all cases, the detection efficiency rather rapidly increases as the SCR widens starting at high uncompensated impurity concentrations (1015 cm−3). In addition, recombination losses in the SCR also increase and ultimately become so significant that the detection efficiency decreases with a further increase in *N*<sup>d</sup> *– N*<sup>a</sup> . The obtained results for the measurements and calculations show that, together with high resistivity, lifetime and mobility of charge carriers, the concentration of uncompensated impurities in the range 1011–1013 cm−3 can be considered also necessary condition for the efficient operation of X/γ-rays detectors based on CdTe and Cd0,9Zn0,1Te [10, 11]. It is the concentration of uncompensated impurities of 1012 cm−3 in CdTe crystals made it possible to obtain 137Cs radioisotope energy spectrum by an Ni/CdTe/Ni diode detector at applied reverse bias voltage of 1200 V with the record values of energy resolution at room temperature (2.8 keV of FWHM at 662 keV) (**Figure 6c**, inset).

semitransparent Schottky contact, the laser radiation is absorbed in a thin near-surface layer

Although all crystals had high resistivity and minority carrier lifetime, the diodes showed significant differences in the registration of spectra from 137Cs (662 кеВ), 133Ba (356 кеВ), 57Co (122 кеВ), 241Am (59 кеВ), 55Fe (5,9 кеВ) isotopes. The CdTe detector was a high resolution detector, however the CdZnTe registered the spectra but with lower resolution (**Figure 6a**). At first glance it seems unclear as CdTe inferior in characteristics Cd0,9Zn0,1Te. Obviously, the detecting properties of the diode structure are influenced by other characteristics of the material. There is an assumption that such a parameter is the SCR width of a Schottky diode, which in a compensated semiconductor can be significant. Indeed, in the detector with Schottky diode, the SCR itself is an active area of the detector, and its width, of course, is one of the most important parameters. Therefore, in compensated semiconductor in addition to the high resistivity and long carrier lifetimes that is necessary for high detection efficiency, another mandatory requirement to the concentration of uncompensated impurities in the material

The detection efficiency spectra of CdTe-based crystals with Schottky diode taking into

**Figure 6b** shows the detection efficiency spectra of CdTe (Cd0,9Zn0,1Te) crystals with Schottky diode at the voltage *V* = 400 V applied to the detector and different concentrations of uncom-

to 1010 cm−3, the detection efficiency of 55Fe (*hν* = 5.9 kеV) and 241Am (*hν* = 59.5 kеV) isotopes vary almost by 3 and 2 orders of magnitude, respectively. At the same decreasing *N*<sup>d</sup> *- N*<sup>a</sup>

the detection efficiency of 57Co (*hν* = 122 kеV) isotope varies within one order of magnitude and the detection efficiency of 133Ba (356 kеV) and 137Cs (662 kеV) isotopes vary relatively

are described by a curve with maximum (**Figure 6c**) [6, 10, 11, 25]. As seen, in all cases, the detection efficiency rather rapidly increases as the SCR widens starting at high uncompensated impurity concentrations (1015 cm−3). In addition, recombination losses in the SCR also increase and ultimately become so significant that the detection efficiency decreases with a

that, together with high resistivity, lifetime and mobility of charge carriers, the concentration of uncompensated impurities in the range 1011–1013 cm−3 can be considered also necessary condition for the efficient operation of X/γ-rays detectors based on CdTe and Cd0,9Zn0,1Te [10, 11]. It is the concentration of uncompensated impurities of 1012 cm−3 in CdTe crystals made it possible to obtain 137Cs radioisotope energy spectrum by an Ni/CdTe/Ni diode detector at applied reverse bias voltage of 1200 V with the record values of energy resolution at room

*<sup>W</sup> <sup>α</sup>*CdTe exp(−*α*CdTe *<sup>x</sup>*)dx <sup>+</sup> *<sup>L</sup>* \_\_\_\_\_*<sup>n</sup>*

1 + *Ln*

in the material. The results clearly illustrate the fact that the spectra

. The obtained results for the measurements and calculations show

is changed. If *N*<sup>d</sup> *– N*<sup>a</sup>

exp(−*W*)

). (7)

decreases from 1014

) for all the isotopes

,

of the SCR and lifetimes of carriers are relatively short (10–20 ns).

(which determines the SCR width of a Schottky diode) is substantiated.

account the drift and diffusion components can be expressed as [25].

weak. An important feature of the results is that the dependences *η*(*N*d–*N*<sup>a</sup>

temperature (2.8 keV of FWHM at 662 keV) (**Figure 6c**, inset).

*W <sup>λ</sup>n*)](<sup>∫</sup> 0

*<sup>η</sup>* <sup>=</sup> *<sup>λ</sup>*\_\_*<sup>n</sup>*

40 New Trends in Nuclear Science

pensated donors *N*<sup>d</sup> *– N*<sup>a</sup>

further increase in *N*<sup>d</sup> *– N*<sup>a</sup>

*W*[

<sup>1</sup> <sup>−</sup> exp(−\_\_

of *η*(*hν*) can significantly be modified when *N*<sup>d</sup> *– N*<sup>a</sup>

To determine the concentration of uncompensated impurities in crystals of CdTe and Cd0,9Zn0,1Te, we compared the detection efficiency with irradiation of the crystal by the Оhmic contact side and the Schottky contact side. In the high-energy region of the spectrum, in which the absorption coefficient for X/γ-rays (*α*γ) is small, excitation occurs virtually uniformly over the entire crystal volume and the detection efficiency for a crystal with the Schottky contact is independent of which side of the detector is irradiated, the side of the Schottky contact or the side of the Ohmic contact. This is confirmed experimentally. If a CdTe detector with a Schottky contact is subjected to the radiation of the 137Cs isotope, the peak height at the photon energy 662 keV (*α*<sup>γ</sup> ≈ 0.1 cm−1) is practically the same when different sides of the sample are irradiated, the side of the Schottky contact or the side of the Ohmic contact. If the isotope 55Fe is used (*α*<sup>γ</sup> ≈ 4000 cm−1), the peak height in the case of irradiation of the Schottky contact side is by two orders of magnitude, than that in the case of irradiation of the Ohmic contact side, as is shown in **Figure 6a** (inset) [11]. This is accounted for by the fact that, at *α*<sup>γ</sup> ≈ 4000 cm−1, the effective depth of radiation penetration into the material is smaller than 1 μm, therefore, in the case of irradiation of the Ohmic contact side, a significant portion of electrons, which appeared as a result of γ-photon absorption, do not reach the SCR by diffusion [17]. Evidently, in this case, the peak height greatly depends on the SCR width and, consequently, on the concentration of uncompensated impurities in the semiconductor, which can be used to determine the value of *N*<sup>d</sup> *– N*<sup>a</sup> . Thus, there is a significant difference between the concentrations of uncompensated impurities in CdTe and Cd0,9Zn0,1Te crystals, which are used in the fabrication of X/γ-rays detectors. The concentration of uncompensated impurities is ~(1–3) × 1012 cm−3 for CdTe crystals and (1–5) × 10<sup>8</sup> cm−3 (i.e., four orders of magnitude lower) for Cd0,9Zn0,1Te crystals [11]. The low concentration of uncompensated impurities (10<sup>8</sup> –109 сm−3) is the reason for the unsatisfactory detectivity of Cd0,9Zn0,1Te detectors regardless of a fully acceptable resistivity of crystals (> 109 Ω·сm) and the lifetime of charge carriers (> 10−6 s) [10].

**Figure 6.** (a) Spectra of 133Ba isotope taken with Schottky diode detectors based on CdTe and at *V* = 500 V. The inset shows the emission spectra of the 55Fe isotope measured by a CdTe detector with a Schottky contact under irradiation of different sides of the sample. (b) Detection efficiency spectra of CdTe-based detector with Schottky diode calculated for different concentrations of uncompensated donors *N*d – *N*<sup>a</sup> . (c) Normalized detection efficiency of different isotopes as a function of *N*d – *N*<sup>a</sup> . The inset shows the typical 137Cs radioisotope energy spectrum detected by an Ni/CdTe/Ni diode detector.
